Method of manufacturing a semiconductor device having a silicide film

ABSTRACT

A method of manufacturing a semiconductor device includes implanting germanium ions into a selected portion of a semiconductor region containing at least silicon, forming P-type and N-type diffusion layers in the semiconductor region, and forming a suicide film which extends from the N type diffusion layer over to the boundary region and the P-type diffusion layer. A boundary region between the P-type and N-type diffusion layers is formed in the selected portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2002-235073, filed Aug. 12,2002, the entire contents of which are incorporated herein by reference.

This application is a Continuation application of U.S. Ser. No.10/716,142, now filed Nov. 19, 2003, now U.S. Pat. No. 6,841,429,allowed, which is a Divisional application of Ser. No. 10/241,488 filedSep. 12, 2002, now U.S. Pat. No. 6,677,660.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor device and a method ofmanufacturing the same and more particularly to a semiconductor devicehaving a silicide film and a method of manufacturing the same.

2. Description of the Related Art

As the performance of a MOS transistor is more enhanced, it becomes morepopular to form the gate electrode in a silicide form in order to reducethe parasitic resistive component thereof. For an integrated circuitsuch as an SRAM which requires extremely high integration density, atransistor structure in which the gates of an NMOS transistor and a PMOStransistor are used as one Si gate pattern and a junction portionbetween an N+ diffusion layer and P+ diffusion layer in the Si gatepattern is short-circuited by use of a silicide film is formed.

In a case where the N+ diffusion layer and P+ diffusion layer are thusformed in the same Si gate pattern, normally, the Si gate pattern isformed with a resist mask and N-type and P-type impurities areselectively ion-implanted. At this time, the N+ diffusion layer and P+diffusion layer may be superposed depending on the alignment position ofthe resist mask and an impurity mixture region in which N-type andP-type impurities exist in a mixed form may be formed in the Si gatepattern in some cases. The thickness of a natural oxide film formed onthe surface of the impurity mixture region is different from thethickness of a natural oxide film formed on the surface of the N+diffusion layer and the thickness of a natural oxide film formed on thesurface of the P+ diffusion layer.

Further, it is known that the natural oxide film formed on the surfaceof the P+ diffusion layer is more difficult to remove than the naturaloxide film formed on the surface of the N+ diffusion layer. Morespecifically, since the concentration of holes in the natural oxide filmor oxide film formed on the surface of the P+ diffusion layer becomeshigher, it is difficult to completely remove the oxide film.

Reference document: Sato et al. “Study of HF-Treated Heavily-Doped SiSurface Using Contact Angle Measurements” Jpn. J. Appl. Phys. Vol. 33(1994), pp 6508 to 6513.

When a silicide film is formed on the surface of the Si gate pattern, astep of removing the natural oxide film from the surface of the Si gatepattern is provided as the preprocessing step. However, if the thicknessof the natural oxide film formed on the surface of the Si gate patternvaries and the difficulty in removing the natural oxide film varies, thenatural oxide film cannot be completely removed in the preprocessingstep and may be partly left behind on the surface of the Si gate patternin some cases. The thus remaining region of the natural oxide film willobstruct the silicidation reaction between Si and metal. As a result,the resistance may increase in the remaining region of the natural oxidefilm in the Si gate pattern and an “open” defect may occur. Next, anexample of the problem is explained.

FIGS. 33A to 33E are cross sectional views showing a manufacturingmethod of the conventional semiconductor device in the order of themanufacturing steps and particularly showing a case wherein an impuritymixture region is formed in the Si gate pattern.

First, as shown in FIG. 33A, a P+ diffusion layer 104, N+ diffusionlayer 105 and N+/P+ mixed layer 107 are formed in an Si gate pattern101. Further, a natural oxide film 110 is formed on the surface of theSi gate pattern 101 and, particularly, the film thickness t107 of thenatural oxide film formed on the surface of the impurity mixture region107 is different from the film thickness t104 of the natural oxide filmformed on the surface of the P+ diffusion layer 104 and the filmthickness t105 of the natural oxide film formed on the surface of the N+diffusion layer. Specifically, the film thickness t107 is larger thanthe film thickness t104 and the film thickness t107.

Next, as shown in FIG. 33B, the natural oxide film 110 is etched by awet etching process using hydrofluoric acid or the like. At this time,it is assumed that the natural oxide film 110 is partly left behind onthe surface of the impurity mixture region 107.

Then, as shown in FIG. 33C, a metal film 111 is formed on the Si gatepattern 101 with the natural oxide film 110 partly left behind thereon.

After this, as shown in FIG. 33D, the heat treatment is performed tocause a reaction between the Si gate pattern 101 and the metal film 111so as to form a silicide film 109. At this time, since the reaction isdifficult to occur on the natural oxide film 110, the silicide film 109is not practically formed on the natural oxide film 110.

Next, as shown in FIG. 33E, a non-reacted portion of the metal film 111is removed. Thus, the Si gate pattern 101 whose surface is formed in asilicide form can be obtained.

However, since the silicide film 109 is not practically formed on theimpurity mixture region 107, the silicide film 109 is divided on aboundary portion 106 between the P+ diffusion layer 104 and the N+diffusion layer 105. As a result, a junction portion between the P+diffusion layer 104 and the N+ diffusion layer 105 cannot beshort-circuited by use of the silicide film 109. For example, this maybe a cause of the “open” defect.

FIGS. 34A to 34E are cross sectional views showing another manufacturingmethod of the conventional semiconductor device in the order of themanufacturing steps and particularly showing a case wherein a naturaloxide film is left behind on the surface of a P+ diffusion layer.

First, as shown in FIG. 34A, a P+ diffusion layer 104 and N+ diffusionlayer 105 are formed in an Si gate pattern 101 and a natural oxide film110 is formed on the surface of the Si gate pattern 101.

Next, as shown in FIG. 34B, the natural oxide film 110 is etched by awet etching process using hydrofluoric acid or the like. At this time,it is assumed that the natural oxide film 110 is partly left behind onthe surface of the P+ diffusion layer 104.

Then, as shown in FIG. 34C, a metal film 111 is formed on the Si gatepattern 101 with the natural oxide film 110 partly left behind thereon.

After this, as shown in FIG. 34D, the heat treatment is performed tocause a reaction between the Si gate pattern 101 and the metal film 111so as to form a silicide film 109. At this time, as explained withreference to FIG. 33D, the silicide film 109 is not practically formedon the natural oxide film 110.

Next, as shown in FIG. 34E, a non-reacted portion of the metal film 111is removed. Thus, the Si gate pattern 101 whose surface is formed in asilicide form can be obtained.

However, since the silicide film 109 is not practically formed on thenatural oxide film 110, the silicide film 109 is divided on the P+diffusion layer 104. As a result, the resistance will increase on aregion of the P+ diffusion layer 104 on which the natural oxide film 110is left behind.

The problems caused by leaving behind the natural oxide film 10 on theSi gate pattern 101 can be solved by increasing an etching amount of thenatural oxide film 110 in the steps shown in FIGS. 33B and 34B, forexample. However, if an etching amount of the natural oxide film 110 isincreased, excessive etching will occur in a portion of the integratedcircuit, for example, in the element isolation region. Next, a typicalexample of a problem caused by the excessive etching is explained below.

FIGS. 35 to 39 are cross sectional views showing a manufacturing methodof the conventional semiconductor device in the order of themanufacturing steps and particularly showing a salicide process.

First, as shown in FIG. 35, an element isolation region 122 is formed onthe surface region of an N-type well region 121 to define an elementregion 123. In the element region 123, P+ diffusion layers 124 and P−diffusion layers 125 which act as the source/drain regions of an MOSFETare formed. The P− diffusion layer 125 is a region called an LDD(Lightly Doped Drain) region or extension region in the MOSFET with theLDD structure. A gate insulating film 126 is formed on a channel regionbetween the P− diffusion layers 125 and a gate electrode 127 is formedon the gate insulating film 126. The gate electrode 127 is formed ofsilicon having P-type impurity doped therein and, for example,corresponds to the P+ diffusion layer 104 of the Si gate pattern 101shown in FIGS. 33A and 34A. A side wall insulating film 128 is formed onthe side walls of the gate electrode 127 and on the P− diffusion layers125. The side wall insulating film 128 is a silicon oxide film. Anatural oxide film 110 is formed on the surfaces of the P+ diffusionlayers 124 and on the surface of the gate electrode 127.

Next, as shown in FIG. 36, the natural oxide film 110 is etched by a wetetching process using hydrofluoric acid or the like. In the etchingprocess, it is assumed that an etching amount of the natural oxide film110 is increased to completely remove the natural oxide film.110 fromthe surface of the gate electrode 127 and the surfaces of the P+diffusion layers 124. At this time, excessive etching occurs in theelement isolation region 122 and side wall insulating film 128 to reducethe film thicknesses thereof. In this case, if the film thickness of theelement isolation region 122 is extremely reduced, the upper surface ofthe etched element isolation region 122 becomes lower than the junctionbetween the P+ diffusion layer 124 and the N-type well region 121 asindicated by a reference symbol 130 so as to expose a portion of theN-type well region 121 in some cases.

Then, as shown in FIG. 37, a metal film 111 is formed on the structurein which the portion of the N-type well region 121 is exposed.

After this, as shown in FIG. 38, the heat treatment is performed tocause a reaction between Si of the gate electrode 127 and element region123 and the metal film 111 so as to form a silicide film 109.

Next, as shown in FIG. 39, a non-reacted portion of the metal film 111is removed. Thus, the surface of the gate electrode 127 and the surfaceof the P+ diffusion layer 124 are formed in a silicide form.

However, since the film thickness of the element isolation region 122 isreduced and the N-type well region 121 is partly exposed, the silicidefilm 109 is formed to extend over the P+ diffusion layer 124 and N-typewell region 121. As a result, the P+ diffusion layer 124 and the N-typewell region 121 are short-circuited via the silicide film 109. Then, aproblem that a junction leak between the P+ diffusion layer 124 and theN-type well region 121 is increased or the MOSFET will not be operatedwill occur.

BRIEF SUMMARY OF THE INVENTION

A method of manufacturing a semiconductor device according to a firstaspect of the present invention comprises implanting germanium ions intoa selected portion of a semiconductor region containing at leastsilicon, forming P-type and N-type diffusion layers in the semiconductorregion with a boundary region between the above diffusion layers beingset in the selected portion, and forming a silicide film which extendsfrom the N-type diffusion layer over to the boundary region and theP-type diffusion layer.

A method of manufacturing a semiconductor device according to a secondaspect of the present invention comprises forming a germaniumlow-concentration region containing germanium of low concentration and agermanium high-concentration region containing germanium of highconcentration in a semiconductor region containing at least silicon,forming P-type and N-type diffusion layers in the semiconductor regionwith a boundary region between the above diffusion layers being set inthe germanium high-concentration region, and forming a silicide filmwhich extends from the N-type diffusion layer over to the boundaryregion and the P-type diffusion layer.

A method of manufacturing a semiconductor device according to a secondaspect of the present invention comprises implanting first germaniumions into a first selected portion of a semiconductor region containingat least silicon, implanting second germanium ions into a secondselected portion of the semiconductor region, the dose amount of thesecond germanium ions being smaller than the dose amount of the firstgermanium ions, forming P-type and N-type diffusion layers in thesemiconductor region with a boundary region between the above diffusionlayers being set in the first selected portion, and forming a silicidefilm which extends from the N-type diffusion layer over to the boundaryregion and the P-type diffusion layer.

A method of manufacturing a semiconductor device according to a thirdaspect of the present invention comprises implanting germanium ions intoa selected portion of a semiconductor region containing at least siliconand germanium, forming P-type and N-type diffusion layers in thesemiconductor region with a boundary region between the above.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1A is a cross sectional view showing a semiconductor deviceaccording to a first embodiment of the present invention, FIG. 1B is across sectional view showing a semiconductor device according to a firstmodification of the first embodiment of the present invention, and FIG.1C is a cross sectional view showing a semiconductor device according toa second modification of the first embodiment of the present invention;

FIG. 2 is a cross sectional view showing one manufacturing step of thesemiconductor device according to the first embodiment of the presentinvention;

FIG. 3 is a cross sectional view showing one manufacturing step of thesemiconductor device according to the first embodiment of the presentinvention;

FIG. 4 is a cross sectional view showing one manufacturing step of thesemiconductor device according to the first embodiment of the presentinvention;

FIG. 5 is a cross sectional view showing one manufacturing step of thesemiconductor device according to the first embodiment of the presentinvention;

FIG. 6 is a cross sectional view showing one manufacturing step of thesemiconductor device according to the first embodiment of the presentinvention;

FIG. 7 is a cross sectional view showing one manufacturing step of thesemiconductor device according to the first embodiment of the presentinvention;

FIG. 8 is a cross sectional view showing one manufacturing step of thesemiconductor device according to the first embodiment of the presentinvention;

FIG. 9 is a cross sectional view showing one manufacturing step of thesemiconductor device according to the first embodiment of the presentinvention;

FIG. 10 is a cross sectional view showing one manufacturing step of thesemiconductor device according to the first embodiment of the presentinvention;

FIG. 11 is a cross sectional view showing one manufacturing step of thesemiconductor device according to the first embodiment of the presentinvention;

FIG. 12 is a cross sectional view showing one manufacturing step of thesemiconductor device according to the first embodiment of the presentinvention;

FIG. 13 is a cross sectional view showing one manufacturing step of thesemiconductor device according to the first embodiment of the presentinvention;

FIG. 14A is a plan view showing a semiconductor device according to asecond embodiment of the present invention, FIG. 14B is a crosssectional view taken along the 14B—14B line of FIG. 14A, FIG. 14C is across sectional view taken along the 14C—14C line of FIG. 14A, and FIG.14D is a cross sectional view taken along the 14D—14D line of FIG. 14A;

FIG. 15A is a plan view showing one manufacturing method of thesemiconductor device according to the second embodiment of the presentinvention, FIG. 15B is a cross sectional view taken along the 15B—15Bline of FIG. 15A, FIG. 15C is a cross sectional view taken along the15C—15C line of FIG. 15A, and FIG. 15D is a cross sectional view takenalong the 15D—15D line of FIG. 15A;

FIG. 16A is a plan view showing one manufacturing method of thesemiconductor device according to the second embodiment of the presentinvention, FIG. 16B is a cross sectional view taken along the 16B—16Bline of FIG. 16A, FIG. 16C is a cross sectional view taken along the16C—16C line of FIG. 16A, and FIG. 16D is a cross sectional view takenalong the 16D—16D line of FIG. 16A;

FIG. 17A is a plan view showing one manufacturing method of thesemiconductor device according to the second embodiment of the presentinvention, FIG. 17B is a cross sectional view taken along the 17B—17Bline of FIG. 17A, FIG. 17C is a cross sectional view taken along the17C—17C line of FIG. 17A, and FIG. 17D is a cross sectional view takenalong the 17D—17D line of FIG. 17A;

FIG. 18A is a plan view showing one manufacturing method of thesemiconductor device according to the second embodiment of the presentinvention, FIG. 18B is a cross sectional view taken along the 18B—18Bline of FIG. 18A, FIG. 18C is a cross sectional view taken along the18C—18C line of FIG. 18A, and FIG. 18D is a cross sectional view takenalong the 18D—18D line of FIG. 18A;

FIG. 19A is a plan view showing one manufacturing method of thesemiconductor device according to the second embodiment of the presentinvention, FIG. 19B is a cross sectional view taken along the 19B—19Bline of FIG. 19A, FIG. 19C is a cross sectional view taken along the19C—19C line of FIG. 19A, and FIG. 19D is a cross sectional view takenalong the 19D—19D line of FIG. 19A;

FIG. 20A is a plan view showing one manufacturing method of thesemiconductor device according to the second embodiment of the presentinvention, FIG. 20B is a cross sectional view taken along the 20B—20Bline of FIG. 20A, FIG. 20C is a cross sectional view taken along the20C—20C line of FIG. 20A, and FIG. 20D is a cross sectional view takenalong the 20D—20D line of FIG. 20A;

FIG. 21A is a plan view showing one manufacturing method of thesemiconductor device according to the second embodiment of the presentinvention, FIG. 21B is a cross sectional view taken along the 21B—21Bline of FIG. 21A, FIG. 21C is a cross sectional view taken along the21C—21C line of FIG. 21A, and FIG. 21D is a cross sectional view takenalong the 21D—21D line of FIG. 21A;

FIG. 22A is a plan view showing one manufacturing method of thesemiconductor device according to the second embodiment of the presentinvention, FIG. 22B is a cross sectional view taken along the 22B—22Bline of FIG. 22A, FIG. 22C is a cross sectional view taken along the22C—22C line of FIG. 22A, and FIG. 22D is a cross sectional view takenalong the 22D—22D line of FIG. 22A;

FIG. 23A is a plan view showing one manufacturing method of thesemiconductor device according to the second embodiment of the presentinvention, FIG. 23B is a cross sectional view taken along the 23B—23Bline of FIG. 23A, FIG. 23C is a cross sectional view taken along the23C—23C line of FIG. 23A, and FIG. 23D is a cross sectional view takenalong the 23D—23D line of FIG. 23A;

FIG. 24A is a plan view showing one manufacturing method of thesemiconductor device according to the second embodiment of the presentinvention, FIG. 24B is a cross sectional view taken along the 24B—24Bline of FIG. 24A, FIG. 24C is a cross sectional view taken along the24C—24C line of FIG. 24A, and FIG. 24D is a cross sectional view takenalong the 24D—24D line of FIG. 24A;

FIG. 25A is a plan view showing one manufacturing method of thesemiconductor device according-to the second embodiment of the presentinvention, FIG. 25B is a cross sectional view taken along the 25B—25Bline of FIG. 25A, FIG. 25C is a cross sectional view taken along the25C—25C line of FIG. 25A, and FIG. 25D is a cross sectional view takenalong the 25D—25D line of FIG. 25A;

FIG. 26A is a plan view showing one manufacturing method of thesemiconductor device according to the second embodiment of the presentinvention, FIG. 26B is a cross sectional view taken along the 26B—26Bline of FIG. 26A, FIG. 26C is a cross sectional view taken along the26C—26C line of FIG. 26A, and FIG. 26D is a cross sectional view takenalong the 26D—26D line of FIG. 26A;

FIG. 27A is a plan view showing one manufacturing method of thesemiconductor device according to the second embodiment of the presentinvention, FIG. 27B is a cross sectional view taken along the 27B—27Bline of FIG. 27A, FIG. 27C is a cross sectional view taken along the27C—27C line of FIG. 27A, and FIG. 27D is a cross sectional view takenalong the 27D—27D line of FIG. 27A;

FIG. 28A is a plan view showing one manufacturing method of thesemiconductor device according to the second embodiment of the presentinvention, FIG. 28B is a cross sectional view taken along the 28B—28Bline of FIG. 28A, FIG. 28C is a cross sectional view taken along the28C—28C line of FIG. 28A, and FIG. 28D is a cross sectional view takenalong the 28D—28D line of FIG. 28A;

FIG. 29A is a plan view showing a semiconductor device according to athird embodiment of the present invention, FIG. 29B is a cross sectionalview taken along the 29B—29B line of FIG. 29A, FIG. 29C is a crosssectional view taken along the 29C—29C line of FIG. 29A, and FIG. 29D isa cross sectional view taken along the 29D—29D line of FIG. 29A;

FIG. 30A is a plan view showing the semiconductor device according tothe third embodiment of the present invention, and FIG. 30B is a crosssectional view taken along the 30B—30B line of FIG. 30A;

FIGS. 31A, 31B and 31C are cross sectional views showing a firstmodification of a forming method of a Ge high-concentration region;

FIGS. 32A and 32B are cross sectional views showing a secondmodification of the forming method of a Ge high-concentration region;

FIGS. 33A, 33B, 33C, 33D and 33E are cross sectional views showing amanufacturing method of the conventional semiconductor device in theorder of the manufacturing steps;

FIGS. 34A, 34B, 34C, 34D and 34E are cross sectional views showing amanufacturing method of the conventional semiconductor device in theorder of the manufacturing steps;

FIG. 35 is a cross sectional view showing one manufacturing step of theconventional semiconductor device;

FIG. 36 is a cross sectional view showing one manufacturing step of theconventional semiconductor device;

FIG. 37 is a cross sectional view showing one manufacturing step of theconventional semiconductor device;

FIG. 38 is a cross sectional view showing one manufacturing step of theconventional semiconductor device; and

FIG. 39 is a cross sectional view showing one manufacturing step of theconventional semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

There will now be described embodiments of this invention with referenceto the accompanying drawings. In the following explanation, commonportions are denoted by the same reference symbols throughout thedrawings.

First Embodiment

FIG. 1A is a cross sectional view showing a semiconductor deviceaccording to a first embodiment of the present invention.

As shown in FIG. 1A, silicon (Si) and germanium (Ge) are contained in asemiconductor region, for example, in a semiconductor film 1. Oneexample of the semiconductor film 1 is the gate pattern of a MOSFET. AGe low-concentration region 2 in which germanium is contained with lowconcentration and a Ge high-concentration region 3 in which germanium iscontained with high concentration are formed in the semiconductor film 1in the present embodiment.

Further, in the semiconductor film 1, a P+ diffusion layer 4 and N+diffusion layer 5 are formed. A boundary region 6 between the P+diffusion layer 4 and the N+ diffusion layer 5 is formed in the Gehigh-concentration region 3.

The boundary region 6 in the present embodiment contains an impuritymixture region (N+/P+) 7 containing N-type and P-type impurities. The P+diffusion layer 4 is formed in the Ge high-concentration region 3.Further, a boundary 8 between the Ge low-concentration region 2 and theGe high-concentration region 3 is formed in the N+ diffusion layer 5. Asilicide film 9 is continuously formed on the N+ diffusion layer 5,boundary region 6 and P+ diffusion layer 4. The silicide film 9 is notdivided on the P+ diffusion layer 4.

Next, one example of the manufacturing method of the semiconductordevice according to the first embodiment is explained.

FIGS. 2 to 13 are cross sectional views showing the semiconductor deviceof the first embodiment according to one example of the manufacturingmethod thereof.

First, as shown in FIG. 2, a semiconductor film 1 containing at leastsilicon is formed. In this example, the semiconductor film 1 is a gatepattern and formed on a gate insulating film (not shown). The filmthickness of the semiconductor film 1 can be set to a desired value. Ifan example is daringly given, the film thickness is 100 nm to 200 nm ina case where the semiconductor film 1 is a gate pattern and in thegeneration in which the gate length of the transistor is 100 nm.

Further, germanium (Ge) is contained in the semiconductor film 1 of thepresent example in addition to silicon to form a polycrystalline SiGefilm. A natural oxide film 10 is formed on the surface of thesemiconductor film 1.

Next, as shown in FIG. 3, an oxidation-resistant film 30 is formed onthe semiconductor film 1 with the natural oxide film 10 formed thereon.One example of a material of the oxidation-resistant film 30 is siliconnitride. The film thickness of the oxidation-resistant film 30 can beset to film thickness which may suppress oxidation of the semiconductorfilm 1. For example, the film thickness is set to approximately 10 nm to50 nm. If the film thickness is set to approximately 10 nm to 50 nm, theoxidation-resistant film 30 can be easily separated in the later step.

Next, as shown in FIG. 4, the oxidation-resistant film 30 is patternedby a reactive ion etching (RIE) process using a resist mask, forexample, so as to expose the surface of a portion of the semiconductorfilm 1 which corresponds to a P+ diffusion layer forming region. At thistime, an end portion 31 of the oxidation-resistant film 30 is disposedinside an N+ diffusion layer forming region. The end portion 31 of theoxidation-resistant film 30 will define the boundary between the Gelow-concentration region and the Ge high-concentration region.

After this, as shown in FIG. 5, the surface of the semiconductor film 1is oxidized to form a silicon oxide film 32 with the oxidation-resistantfilm 30 used as a mask. By forming the silicon oxide film 32, the filmthickness t1P of the semiconductor film 1 in the P+ diffusion layerforming region becomes smaller than the film thickness t1N thereof inthe N+ diffusion layer forming region. At this time, the property thatgermanium is difficult to be introduced into the silicon oxide film 32is utilized so that the Ge concentration in the thinned semiconductorfilm 1 can be increased. As a result, the region 2 in which the Geconcentration is low and the region 3 in which the Ge concentration ishigh are formed in the semiconductor film 1.

Next, as shown in FIG. 6, the oxidation-resistant film 30 and siliconoxide film 32 are removed from the semiconductor film 1. At this time,the natural oxide film 10 is also removed as shown in FIG. 6. However,if the surface of the semiconductor film 1 is exposed to oxygen in thelater step, a natural oxide film 10 is formed on the surface of thesemiconductor film 1 again.

After this, as shown in FIG. 7, after the semiconductor film 1 ispatterned into a gate pattern, for example, a resist mask 33 used forion-implantation of P-type impurity is formed on the patternedsemiconductor film 1. Then, P-type impurity ions 34, for example,impurity ions containing boron are implanted into the semiconductor film1 with the resist mask 33 used as a mask. At this time, an attempt ismade to dispose an end portion 35 of the resist mask 33 in the Gehigh-concentration region 3.

Next, as shown in FIG. 8, after the resist mask 33 is removed, a resistmask 36 used for ion-implantation of N-type impurity is formed on thesemiconductor film 1. Then, N-type impurity ions 37, for example,impurity ions containing phosphorus or arsenic are implanted into thesemiconductor film 1 with the resist mask 36 used as a mask. At thistime, an attempt is made to dispose an end portion 38 of the resist mask36 in the Ge high-concentration region 3 like the case of the endportion 35 of the resist mask 33.

After-this, as shown in FIG. 9, for example, the heat treatment isperformed to activate the P-type and N-type impurities implanted intothe semiconductor film 1. As a result, the P+ diffusion layer 4 andimpurity mixture region 7 are formed in the Ge high-concentration region3 of the semiconductor film 1 and the N+ diffusion layer 5 is formed inthe Ge low-concentration region 2.

Next, as shown in FIG. 10, for example, the natural oxide film 10 isetched by a wet etching process using hydrofluoric acid (for example,dilute hydrofluoric acid) or an etchant containing hydrofluoric acid.The step is a preprocessing step of forming a metal film used forforming silicide. At this time, as shown in FIG. 10, since the impuritymixture region 7 is formed inside the Ge high-concentration region 3,the natural oxide film 10 formed on the surface of the impurity mixtureregion 7 can be easily removed in comparison with the process explainedwith reference to FIGS. 33A to 33E, for example. Further, since the P+diffusion layer 4 is formed inside the Ge high-concentration region 3like the impurity mixture region 7, the natural oxide film 10 formed onthe surface of the P+ diffusion layer 4 can be easily removed incomparison with the process explained with reference to FIGS. 34A to34E, for example. The reason why the natural oxide film 10 can be easilyremoved is that the film thickness of the natural oxide film 10 can bemade thinner than in a case where it is formed on silicon containing nogermanium since the natural oxide film 10 is formed on silicon whichcontains germanium or the etching rate with respect to the hydrofluoricacid is increased. Further, a phenomenon that the film thickness of thenatural oxide film 10 is made thin and a phenomenon that the etchingrate with respect to the hydrofluoric acid is increased both become moresignificant as the germanium concentration becomes higher. That is, thefilm thickness of the natural oxide film 10 is made thinner on thesurface of the Ge high-concentration region 3 than on the surface of theGe low-concentration region 2. Further, the etching rate of the naturaloxide film 10 with respect to the hydrofluoric acid is faster on thesurface of the Ge high-concentration region 3 than on the surface of theGe low-concentration region 2. Therefore, the ease of removal of thenatural oxide film 10 formed on the surface of the P+ diffusion layer 4and the surface of the impurity mixture region 7 can be improvedsubstantially to the same degree of the ease of removal of the naturaloxide film 10 formed on the surface of the N+ diffusion layer 5.

Next, as shown in FIG. 11, a metal film 11 used for forming a silicidefilm is formed on the semiconductor film 1 from which the natural oxidefilm 10 is removed. In this case, any type of metal which reacts withsilicon to form silicide can be used, and as an example of the material,titanium (Ti), cobalt (Co), tungsten (W), platinum (Pt), molybdenum(Mo), palladium (Pd), tantalum (Ta) and the like can be given.

As shown in FIG. 12, for example, the heat treatment is performed toreact the semiconductor film 1 with the metal film 11 so as to form asilicide film 9. At this time, in the present embodiment, since thenatural oxide film 10 is removed from the surface of the P+ diffusionlayer 4 and the surface of the impurity mixture region 7, the silicidefilm 9 is continuously formed on the N+ diffusion layer 5, impuritymixture region 7 and P+ diffusion layer 4. Further, the silicide film 9is not divided on the P+ diffusion layer 4.

Next, as shown in FIG. 13, a non-reacted portion of the metal film 11 isremoved. As a result, the semiconductor film 1 whose surface is formedin a silicide form can be obtained.

According to the semiconductor device of the first embodiment, theboundary region 6 between the P+ diffusion layer 4 and the N+ diffusionlayer 5 lies in the Ge high-concentration region 3. Therefore, even ifthe boundary region 6 contains the impurity mixture region 7, thenatural oxide film 10 formed on the surface of the impurity mixtureregion 7 can be more easily removed in comparison with a case of thedevice shown in FIGS. 33A to 33E. As a result, the silicide film 9 canbe more uniformly formed on the semiconductor film 1 and the junctionportion between the P+ diffusion layer 4 and the N+ diffusion layer 5can be more stably short-circuited by use of the silicide film 9. Thus,the probability that the “open” defect occurs can be more suppressed,for example.

The P+ diffusion layer 4 lies within the Ge high-concentration region 3.Therefore, the natural oxide film 10 formed on the surface of the P+diffusion layer 4 can be more easily removed in comparison with a caseof the device shown in FIGS. 34A to 34E, for example. As a result, theprobability that the silicide film 9 is divided on the P+ diffusionlayer 4 can be more suppressed. Thus, an increase in the resistance onthe P+ diffusion layer 4 can be suppressed, for example.

Further, it becomes easier to remove the natural oxide film 10 formed onthe surface of the impurity mixture region 7 and the natural oxide film10 formed on the surface of the P+ diffusion layer 4. The ease ofremoval of the respective natural oxide films can be improvedsubstantially to the same degree of the ease of removal of the naturaloxide film 10 formed on the surface of the N+ diffusion layer 5, forexample. Therefore, a variation in the ease of removal of the naturaloxide films 10 can be made small and, for example, it is not necessaryto increase an etching amount of the natural oxide film 10. As a result,problems caused by excessive etching, for example, a problem that aninsulated gate field effect transistor will not be operated or a problemthat the junction leak will increase as explained with reference toFIGS. 35 to 39 can be solved.

With the above semiconductor device, it becomes easy to enhance themanufacturing yield and reduce variation in the characteristics of theelements.

Further, another advantage can be attained by making the germaniumconcentration in the P+ diffusion layer 4 higher than the germaniumconcentration in the N+ diffusion layer.

For example, in an N-channel insulated gate field effect transistor(which is hereinafter briefly referred to as an NMOS for convenience) inwhich the N+ diffusion layer 5 is formed in the gate electrode, theactivation rate of N-type impurity contained in the N+ diffusion layer 5becomes maximum when the germanium concentration in the gate electrodeis approximately 30 mol %.

On the other hand, in a P-channel insulated gate field effect transistor(which is hereinafter briefly referred to as a PMOS for convenience) inwhich the P+ diffusion layer 4 is formed in the gate electrode, theactivation rate of P-type impurity contained in the P+ diffusion layer 4becomes higher as the germanium concentration in the gate electrode ishigher.

That is, in the semiconductor device according to the first embodiment,the germanium concentration of the semiconductor film 1 lying in aregion in which the NMOS is formed is set to such a value that theactivation rate of N-type impurity contained in the N+ diffusion layer 5will become maximum. Then, the germanium concentration of thesemiconductor film 1 lying in a region in which the PMOS is formed isset to exceed the concentration which causes the activation rate tobecome maximum. One example of the concentration which causes theactivation rate to become maximum is approximately 30 mol % as describedabove.

By thus setting the concentration, both of the activation rate of N-typeimpurity contained in the diffusion layer formed in the gate electrodeand the activation rate of P-type impurity can be enhanced. If theactivation rates of the impurities are enhanced, a depletion layerformed in the gate electrode becomes thin and the “apparent thickness”of the gate insulating film obtained by adding the thickness of the gateinsulating film formed under the gate electrode to the thickness of thedepletion layer can be made thin. If the “apparent thickness” of thegate insulating film becomes thin, for example, an advantage that theON-OFF characteristic in the insulated gate field effect transistor canbe improved can be attained and it is effective to optimize the elementcharacteristic.

FIG. 1B is a cross sectional view showing a semiconductor deviceaccording to a first modification of the first embodiment of the presentinvention and FIG. 1C is a cross sectional view showing a semiconductordevice according to a second modification of the first embodiment of thepresent invention.

In the first embodiment, the boundary region 6 contains the impuritymixture region 7. However, the impurity mixture region 7 may not becontained in the boundary region 6.

For example, as shown in FIG. 1B, a contact region 7′ in which the P+diffusion layer 4 and the N+ diffusion layer 5 are formed in contactwith each other may be contained in the boundary region 6.

Further, for example, as shown in FIG. 1C, an undoped region 7″containing none of P-type and N-type impurities may be contained in theboundary region 6.

Second Embodiment

The second embodiment is associated with an example of a case whereinthe first embodiment is applied to a CMOS semiconductor device.

FIG. 14A is a plan view showing a semiconductor device according to thesecond embodiment of the present invention, FIG. 14B is a crosssectional view taken along the 14B—14B line of FIG. 14A, FIG. 14C is across sectional view taken along the 14C—14C line of FIG. 14A, and FIG.14D is a cross sectional view taken along the 14D—14D line of FIG. 14A.

The semiconductor device according to the second embodiment is explainedaccording to one example of the manufacturing method below.

FIGS. 15A to 15D, . . . , FIGS. 28A to 28D are plan views or crosssectional views showing the semiconductor device of the first embodimentaccording to one example of the manufacturing method.

First, as shown in FIGS. 15A to 15D, a P-type semiconductor region 20 inwhich a first transistor is to be formed and an N-type semiconductorregion 21 in which a second transistor is to be formed are formed on asubstrate 19. In this example, the substrate 19 is a semiconductorsubstrate, for example, a P-type or N-type silicon substrate. The P-typesemiconductor region 20 is a P-type well and the N-type semiconductorregion 21 is an N-type well. Then, an element isolation region 22 isformed in the surface regions of the P-type well and N-type well todefine element regions 23 in the P-type well and N-type well. Oneexample of the material of the element isolation region 22 is silicondioxide.

Next, as shown in FIGS. 16A to 16D, the surfaces of the element regions23 are oxidized to form gate insulating films 26. After this, asemiconductor film 1 is formed on the element isolation region 22 andgate insulating films 26. Like the first embodiment, silicon andgermanium are contained in the semiconductor film 1 of this example.

Then, as shown in FIGS. 17A to 17D, an oxidation-resistant film 30 isformed on the semiconductor film 1. One example of the material of theoxidation-resistant film 30 is silicon nitride like the case of thefirst embodiment.

Next, as shown in FIGS. 18A to 18D, the oxidation-resistant film 30 ispatterned to expose the surface of a portion of the semiconductor film 1which corresponds to a P+ diffusion layer forming region. At this time,an end portion 31 of the oxidation-resistant film 30 is disposed insidean N+ diffusion layer forming region like the case of the firstembodiment.

After this, as shown in FIGS. 19A to 19D, the surface of thesemiconductor film 1 is oxidized to form a silicon oxide film 32 withthe oxidation-resistant film 30 used as a mask. As a result, like thefirst embodiment, a region 2 in which the Ge concentration is low and aregion 3 in which the Ge concentration is high are formed on thesemiconductor film 1.

Next, as shown in FIGS. 20A to 20D, the oxidation-resistant film 30 andsilicon oxide film 32 are removed from the semiconductor film 1.

After this, as shown in FIGS. 21A to 21D, the semiconductor film 1 ispatterned into a gate pattern of a gate electrode 27. Then, P-type andN-type impurity ions used to form extensions are respectively implantedinto the N-type semiconductor region 21 and P-type semiconductor region20 while the gate electrode 27, element isolation region 22 and resistmask which is not shown in the drawing are used as a mask.

Next, as shown in FIGS. 22A to 22D, insulator such as silicon dioxide isdeposited on the structure shown in FIGS. 21A to 21D to form aninsulating film. Then, the insulating film is subjected to ananisotropic etching process to form a side wall insulating film 28 onthe side walls of the gate electrode 27.

After this, as shown in FIGS. 23A to 23D, a resist mask 33 is formed ona portion of the gate electrode 27 which lies above the P-typesemiconductor region 20 and on the side wall insulating film 28, P-typesemiconductor region 20 and element isolation region 22. Then, P-typeimpurity ions 34 are implanted into the gate electrode 27 and N-typesemiconductor region 21 with the resist mask 33 used as a mask. At thistime, an end portion 35 of the resist mask 33 is disposed in the Gehigh-concentration region 3 like the case of the first embodiment.

Next, as shown in FIGS. 24A to 24D, after the resist mask 33 is removed,a resist mask 36 is formed on a portion of the gate electrode 27 whichlies above the N-type semiconductor region 21 and on the side wallinsulating film 28, N-type semiconductor region 21 and element isolationregion 22. Then, N-type impurity ions 37 are implanted into the gateelectrode 27 and P-type semiconductor region 20 with the resist mask 36used as a mask. At this time, an end portion 38 of the resist mask 36 isdisposed in the Ge high-concentration region 3 like the case of thefirst embodiment.

Next, as shown in FIGS. 25A to 25D, for example, the heat treatment isperformed to activate the P-type and N-type impurities implanted intothe P-type semiconductor region 20, N-type semiconductor region 21 andgate electrode 27. As a result, a P+ diffusion layer 4 and impuritymixture region 7 are formed in the Ge high-concentration region 3 of thegate electrode 27 and an N+ diffusion layer 5 is formed in the Gelow-concentration region 2. Further, on the P-type semiconductor region20, N+ diffusion layers 24N which act as the source and drain regions ofthe NMOS and N− diffusion layers 25N which act as the source extensionand drain extension of the NMOS are formed. On the N-type semiconductorregion 21, P+ diffusion layers 24P which act as the source and drainregions of the PMOS and P− diffusion layers 25P which act as the sourceextension and drain extension of the PMOS are formed. In FIGS. 25Ato-25D, natural oxide films 10 formed on the surfaces of the gateelectrode 7, N+ diffusion layers 24N and P+ diffusion layers 24P areshown.

Next, as shown in FIGS. 26A to 26D, for example, the natural oxide film10 is etched by a wet etching process using hydrofluoric acid (forexample, dilute hydrofluoric acid) or an etchant containing hydrofluoricacid. The step is a preprocessing step of forming a metal film used forforming silicide. At this time, as shown in the drawing, since theimpurity mixture region 7 is formed inside the Ge high-concentrationregion 3, the natural oxide film 10 formed on the surface of theimpurity mixture region 7 can be easily removed like the case of thefirst embodiment. Further, since the P+ diffusion layer 4 is formedinside the Ge high-concentration region 3, the natural oxide film 10formed on the surface of the P+ diffusion layer 4 can be easily removedlike the case of the first embodiment. Further, since the natural oxidefilm 10 can be easily removed, it is unnecessary to increase the etchingamount of the natural oxide film 10 unlike the case of the processexplained with reference to FIGS. 35 to 39. Therefore, a reduction inthe film thickness of the element isolation region 22 and side wallinsulating film 28 can be suppressed in comparison with a case of theprocess explained with reference to FIGS. 35 to 39.

Next, as shown in FIGS. 27A to 27D, a metal film 11 is formed on thesurfaces of the gate electrode 27, side wall insulating film 28, P-typesemiconductor region 20, N-type semiconductor region 21 and elementisolation region 22 from which the natural oxide film 10 is removed.

As shown in FIGS. 28A to 28D, for example, the heat treatment isperformed to react the gate electrode 27, P-type semiconductor region 20and N-type semiconductor region 21 with the metal film 11 so as to forma silicide film 9. At this time, in the present embodiment, since thenatural oxide film 10 is removed from the surfaces of the P+ diffusionlayer 4 and impurity mixture region 7, particularly, the silicide film 9formed on the gate electrode 27 is continuously formed on the N+diffusion layer 5, impurity mixture region 7 and P+ diffusion layer 4.Further, the silicide film 9 is not divided on the P+ diffusion layer 4.

Next, as shown in FIGS. 14A to 14D, a non-reacted portion of the metalfilm 11 is removed. As a result, a CMOS semiconductor device having thegate electrode 27, N+ diffusion layer 24N and P+ diffusion layer 24Pwhose surfaces are formed in a silicide form can be obtained.

In the semiconductor device according to the second embodiment, theboundary region 6 between the P+ diffusion layer 4 and the N+ diffusionlayer 5 is formed in the Ge high-concentration region 3. Further, the P+diffusion layer 4 lies in the Ge high-concentration region 3. Therefore,the same effect as that of the first embodiment can be attained.

Third Embodiment

Like the second embodiment, the third embodiment is associated with anexample of a case wherein the first embodiment is applied to a CMOSsemiconductor device.

FIG. 29A is a plan view showing a semiconductor device according to thethird embodiment of the present invention, FIG. 29B is a cross sectionalview taken along the 29B—29B line of FIG. 29A, FIG. 29C is a crosssectional view taken along the 29C—29C line of FIG. 29A, and FIG. 29D isa cross sectional view taken along the 29D—29D line of FIG. 29A.

As shown in FIGS. 29A to 29D, the third embodiment is similar to thesecond embodiment except that an SiGe substrate containing silicon andgermanium is used as a substrate 19, a Ge low-concentration region 2′containing germanium of low concentration and a Ge high-concentrationregion 3′ containing germanium of high concentration are formed on theSiGe substrate, an N-type semiconductor region 21 on which P+ diffusionlayers 24P are formed is formed in the Ge high-concentration region 3′,and a P-type semiconductor region 20 on which N+ diffusion layers 24Nare formed is formed in the Ge low-concentration region 2′. With theabove structure, a PMOS is formed in the Ge high-concentration region 3′and an NMOS is formed in the Ge low-concentration region 2′.

A natural oxide film formed on the surface of the P+ diffusion layer ismore difficult to remove than a natural oxide film formed on the surfaceof the N+ diffusion layer as described before.

Therefore, in the third embodiment, it is possible to attain anadvantage that the natural oxide film formed on the surface of the P+diffusion layer 24P can be easily removed and a silicide film 9 can beeasily formed on the P+ diffusion layer 24P by forming the N-typesemiconductor region 21 on which the P+ diffusion layers 24P are formedin the Ge high-concentration region 3′.

Thus, the methods explained in the first and second embodiments can beapplied not only to the gate electrodes but also to semiconductoractivation layers, for example, diffusion layers functioning assource/drain regions.

Fourth Embodiment

FIG. 30A is a plan view showing the semiconductor device according tothe fourth embodiment of the present invention and FIG. 30B is a crosssectional view taken along the 30B—30B line of FIG. 30A.

In the third embodiment, the fact that the methods explained in thefirst and second embodiments can be applied to semi-conductor activationlayers, for example, diffusion layers functioning as source/drainregions was explained.

As shown in FIGS. 30A, 30B, in the fourth embodiment, a P+ diffusionlayer 29P is formed on a P-type semiconductor region 20. The P+diffusion layer 29P functions as a contact which is used to apply thesame potential as the source potential of a transistor to the P-typesemiconductor region 20 functioning as a back-gate of the transistor.

A substrate 19 in the present embodiment is an SiGe substrate containingsilicon and germanium. Like the third embodiment, a Ge low-concentrationregion 2′ containing germanium of low concentration and a Gehigh-concentration region 3′ containing germanium of high concentrationare formed in the SiGe substrate. The P+ diffusion layer 29P is formedin the Ge high-concentration region 3′. Further, the P+ diffusion layer29P is formed in contact with the N+ diffusion layer 24N via an impuritymixture region 7′. The impurity mixture region 7′ is formed in the Gehigh-concentration region 3′. A silicide film 9 is continuously formedon the N+ diffusion layer 24N, impurity mixture region 7′ and P+diffusion layer 29P. Further, the silicide film 9 is not divided on theP+ diffusion layer 4.

(Modification of the Method for Forming the Ge High-concentrationRegion)

Next, a modification of the Ge high-concentration region forming methodis explained.

In the first and second embodiments, when the Ge high-concentrationregion 3 is formed, a selected portion of the semiconductor film 1 isoxidized and then the Ge high-concentration region 3 is formed byutilizing the phenomenon that Ge is difficult to be introduced into theoxide film. However, the Ge high-concentration region 3 can be formed byuse of a method other than the above forming method.

(First Modification)

FIGS. 31A, 31B and 31C are cross sectional views showing a firstmodification of the forming method of the Ge high-concentration region.

First, as shown in FIG. 31B, a resist mask 40 is formed on asemiconductor film 1 containing silicon as shown in FIG. 31A. Then,germanium ions 41 are implanted into the semiconductor film 1 with theresist mask 40 used as a mask.

Next, as shown in FIG. 31C, for example, a resist mask 42 is formed on aportion of the semiconductor film 1 into which germanium is implanted.Then, germanium ions 41 which are smaller in dose amount than in a caseof the step shown in FIG. 31B are implanted into the semiconductor film1 with the resist mask 42 used as a mask.

Thus, by implanting germanium ions into the semiconductor film 1 afterchanging the dose amount thereof, the Ge low-concentration region 2 andGe high-concentration region 3 can be formed in the semiconductor film1.

(Second Modification)

FIGS. 32A and 32B are cross sectional views showing a secondmodification of the Ge high-concentration region forming method.

First, as shown in FIG. 32B, a resist mask 40 is formed on asemiconductor film 1 containing silicon as shown in FIG. 32A. Then,germanium ions 41 are implanted into the semiconductor film 1 with theresist mask 40 used as a mask.

Thus, additionally implanting germanium ions into the semiconductor film1, the Ge low-concentration region 2 and Ge high-concentration region 3can be formed in the semiconductor film 1.

As described above, the present invention has been explained by use ofthe first to fourth embodiments, but the present invention is notlimited to the first to fourth embodiments and can be variously modifiedwithout departing from the technical scope thereof.

For example, in the first to fourth embodiments, the insulated gatefield effect transistor is shown as an example, but the presentinvention is not applied only to the insulated gate field effecttransistor. The present invention can also be applied to an activeelement other than the insulated gate field effect transistor, forexample, a bipolar transistor or the like if it is a semiconductordevice having a silicide film. Further, the present invention can beapplied not only to an active element but also to a passive element suchas a capacitor, or diode if it has a silicide film.

In addition, the above embodiments can be independently performed, butit is of course possible to adequately combine and demonstrate theembodiments.

Further, inventions of various stages are contained in the aboveembodiments and can be extracted by adequately combining a plurality ofconstituent elements disclosed in the respective embodiments.

As described above, according to the first to fourth embodiments of thepresent invention, a semiconductor device in which a silicide film canbe easily formed and a manufacturing method thereof can be provided.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A method of manufacturing a semiconductor device comprising:implanting germanium ions into a selected portion of a semiconductorregion containing at least silicon; forming P-type and N-type diffusionlayers in the semiconductor region with a boundary region between theabove diffusion layers being set in the selected portion; and forming asilicide film which extends from the N-type diffusion layer over to theboundary region and the P-type diffusion layer.
 2. The method accordingto claim 1, wherein the P-type diffusion layer is formed in the selectedportion.
 3. The method according to claim 1, wherein a natural oxidefilm is removed before the silicide film is formed.
 4. The methodaccording to claim 3, wherein an etchant containing hydrofluoric acid isused to remove the natural oxide film.
 5. A method of manufacturing asemiconductor device comprising: implanting first germanium ions into afirst selected portion of a semiconductor region containing at leastsilicon; implanting second germanium ions into a second selected portionof the semiconductor region, the dose amount of the second germaniumions being smaller than the dose amount of the first germanium ions;forming P-type and N-type diffusion layers in the semiconductor regionwith a boundary region between the above diffusion layers being set inthe first selected portion; and forming a silicide film which extendsfrom the N-type diffusion layer over to the boundary region and theP-type diffusion layer.
 6. The method according to claim 5, wherein theP-type diffusion layer is formed in the first selected portion.
 7. Themethod according to claim 5, wherein a natural oxide film is removedbefore the silicide film is formed.
 8. The method according to claim 7,wherein an etchant containing hydrofluoric acid is used to remove thenatural oxide film.
 9. A method of manufacturing a semiconductor devicecomprising: implanting germanium ions into a selected portion of asemiconductor region containing at least silicon and germanium; formingP-type and N-type diffusion layers in the semiconductor region with aboundary region between the above diffusion layers being set in theselected portion; and forming a silicide film which extends from the Ntype diffusion layer over to the boundary region and the P-typediffusion layer.
 10. The method according to claim 9, wherein the P-typediffusion layer is formed in the first selected portion.
 11. The methodaccording to claim 9, wherein a natural oxide film is removed before thesilicide film is formed.
 12. The method according to claim 11, whereinan etchant containing hydrofluoric acid is used to remove the naturaloxide film.